From EP 553 266 B1, there is known a method for determining the 3D coordinates of the surface of an object, in which the 3D coordinates of the surface of the object are determined by a 3D measuring device. According to this method, the position and the orientation of a 2D laser scanner are determined by means of an optical tracking system in a high resolution with respect to time, for instance with about 100 images per second. By means of this method, measurement errors of 0.1 to 0.5 mm can be implemented within a measurement volume of about 4 m×4 m×6 m.
In another prior art method, the 3D coordinates of the surface of an object are determined by a camera-based optical 3D measuring device, where the position and the orientation of this 3D measuring device are not determined directly. The 3D measuring device scans individual partial surfaces, for instance within a volume of 1 m×1 m×1 m. Subsequently, the 3D coordinates of the partial surfaces of the object are put together, in particular by means of a calculation method performed in a computer, for instance a PC. To do so, several possibilities exist:
A first possibility consists in putting together the 3D coordinates of the overlap region of adjacent partial surfaces by means of a matching method, which in the specialized literature is also referred to as “registration” or “registration method”. In accordance with this method, partial surfaces, in particular structured partial surfaces, are scanned with an overlap. The distances of the 3D coordinates of two measurements of adjacent partial surfaces are minimized in the overlap region. In terms of software, this is effected by a matching algorithm, in which an error function is determined for points of the overlap region and minimized iteratively. In the case of relatively small, structured surfaces, good results can be achieved therewith. In the case of large objects, however, there can only be achieved measurement errors of 0.2 to 1.0 mm due to the propagation of errors. A second possibility consists in performing a transformation onto reference points. In this case, reference marks are adhered onto the surface of the object. The reference marks are calibrated by means of a measuring machine, in particular a tactile or optical measuring machine. Scanning the surface or a partial surface is effected such that at least three of the measurement marks are detected in such individual measurement, i.e. are scanned photogrammetrically. From the measurement marks thus detected, and from their previously calibrated positions, a transformation can be determined. If this transformation is applied to the individual measurement, the measured values of the surface are obtained. In this method, measurement errors of 0.05 to 0.2 mm can also be achieved with large objects. A major disadvantage, however, consists in that as a result of the application of the reference marks the object to be scanned must be prepared in a time-consuming way. A third possibility consists in combining the first and second possibilities, in order to reduce the effort of the second possibility, but at the same time become more accurate than with the first possibility, i.e. achieve smaller measurement errors.